optimized geometry contain imaginary eigenvalues

[greisen]

Hi,

I am making a geometry optimization of a metal complex - I get convergence and want to calculate the hessian to estimate force constants for some bonds in the complex that I would like to use later on - the problem is that when I do the hessian calculation I get imaginary eigenvalues and I would like to get rid of those. I have tried to reoptimize the complex but still it contains the imaginary eigenvalues. I have tried to include the calculated hessian into geometry optimization
$hessian file=hessian

but I am not sure it does it correctly - I would have imagined that it should have solved the problem.

I use the cosmo-model to mimic the solvent in both the geometry optimization as well as in the numforce calculation.


So how to get rid of these? Any suggestions

Thanks in advance

[Ian K]

You might want to give the 'screwer' module a try: it lets you distort the geometry along a computed mode, so that you could, for example, distort it away from your optimised geometry in the direction of your imaginary mode.

[greisen]

Hi

thanks - I do not think I quiet follow you - I am not familiar with that module. In my case the imaginary eigenvalue is a rotation of a methyl group in my complex  - I thought that I somehow could include the calculated hessian and that would remove the negative curvature of the energy surface?

Thanks

[Ian K]

So, even though you've included the more accurately calculated Hessian, the energy surface is such that the optimisation routine keeps optimising to that saddle-point. I think this can happen when your convergence criteria aren't strict enough - if the imaginary eigenvalue is small, the energy surface at your saddle point behaves quite similarly to that at a genuine local minimum.

The screwer (also known as 'vibrations') tool allows you to distort the geometry along any calculated vibrational mode, including the one corresponding to your imaginary eigenvalue. If you distort it along that one, it could move the geometry out of the 'trap' it's in on the energy surface and allow it to optimise to a proper minimum. The tool is fairly self-explanatory to use - just run it in the directory with the control file and select a mode and a 'temperature' used to determine how much distortion you want; then the distorted coordinates are put in a $newcoord key in the control file. (You then have to copy them into the actual $coord block before it'll use them, as far as I can tell. The documentation on the tool is virtually non-existent.)

Be aware, though, with groups like methyls, I think you can get hindered rotor problems - where the energy surface along the rotational motion isn't sufficiently deep to make the harmonic approximation valid.